The present invention is related to diagnostic fault test systems and circuits, and more particularly to the use of such systems and circuits for testing one or more electrical devices operated in effective on or off states in accordance with received control signals. The present invention has primary application to systems and circuits for sequentially testing a large number of driver stages for solenoids, but can be used in other applications.
Some prior fault diagnostic systems have monitored the entire operation of a complex electrical system and produced fault indications of what type of faults have occurred. U.S. Pat. No. 4,379,990 to Sievers et al., assigned to the same assignee as the present invention, shows one such system. While such a system is advantageous for monitoring the operation of an electrical system, it does not suggest how to rapidly test a number of specific driver stages in both on and off states while also preventing the misdetection of errors caused by monitoring the system during device switching transients. Most of the fault detector circuits in this system are constantly operative and subject to electrical switching transient signals. This system does monitor a single signal and compare it with both a high voltage and low voltage fault threshold, and in this respect, is similar to U.S. Pat. No. 4,316,134 to Balan et al. However, in both cases this comparison apparently occurs continuously and is thus subject to false error detections due to switching transients. In addition, clearly both of these references suggest utilizing separate comparators for comparing a monitored signal with both a high and low threshold. If a number of stages were to be monitored by such systems, this would lead to a substantial number of comparators being utilized and increase the expense and decrease the feasibility of providing such a fault system.
U.S. Pat. No. 4,372,267 to Karlmann et al, also assigned to the same assignee as the present invention, discloses a solenoid fault detection system/circuit which utilizes separate comparators, but now each comparator monitors a different signal and essentially tests for different fault conditions. While this system does contemplate the use of time delay windows to avoid erroneous fault detections due to transient signals, applying the teachings of this patent to test a plurality of solenoid driver stages would produce a complex and costly system having a substantial amount of circuitry used for testing each stage and requiring a substantial amount of time to implement all testing. U.S. Pat. No. 4,764,840 to Petrie et al., assigned to the same assignee as the present invention, discloses a solenoid current controller which monitors a solenoid current sense signal by the utilization of two separate comparators. This system does not really disclose a fault detection system, but merely illustrates how to control maximum and minimum solenoid currents.
U.S. Pat. No. 4,589,401 to Karim et al., discloses a microcomputer which controls the operative state of a number of solenoid driver stages utilized to control engine fuel injection. The system disclosed in this patent contemplates sequential testing of each of the solenoid driver injector circuits for fault conditions. However, in such a system incorrect fault detections may occur due to transient switching signals created by switching a driver stage either immediately prior to or during the time that the driver stage is monitored for a fault condition. Even if this system recognized that it might be necessary to wait a predetermined time after each switching transition before monitoring a driver stage, this would require delaying the fault monitoring by such a waiting time in accordance with each driver stage switching transition. Thus the fault detection system in the '401 patent would not be able to rapidly monitor all solenoid driver stages for faults. In addition, the fault monitoring achieved by the '401 system is essentially limited to determining either (a) if solenoid through current is present or not in accordance with whether or not the driver stage is actuated or (b) if the control signals being supplied to the driver device have failed. Thus, with regard to testing whether or not the driver stage and its associated solenoid have falied, only a limited amount of diagnostic information is available as to identifying what type of failure has occurred. In addition, the '401 system apparently uses multiple integrated circuit comparators each receiving a number of different input signals and comparing them with just one fault threshold level. Thus this system utilizes a substantial amount of circuitry to perform its limited functions.